A new radiological method (dual photon absorptiometry) will be used for assessment of growth and development in normal and abnormal children. Bone mineral content and soft-tissue composition (lean-fat) are measured with high precision and accuracy (less than 2% error) using a congruent radionuclide source of 125-I and 241-Am. This enables objective detection of small differences among children, or in the same child over time. The procedure is innocuous, takes only a few minutes, and has a very small radiation dose (less tha 10 mren) which is wholly confined to a small area on the extremities. Methodological studies are being carried out to validate the absorptiometric method on children and to optimize procedures for field studies. A large sample of children (1000) will be measured and remeasured after a 1-year interval to determine longitudinal growth and to establish norms. Clinical evaluations are being made on selected pediatric patients. We are also examining the bone and tissue hypertrophy associated with exercise in normal children.